Is the following sentence past tense I have borrowed books from library since was in first grade?

The sentence "I have borrowed books from library since was in first grade" is not past tense. It is present perfect tense. The present perfect tense is used to talk about actions or events that started in the past and are still happening now or that have just finished. In this sentence, the action of borrowing books from the library started in the past (in the first grade) and is still happening now. Therefore, the correct tense is present perfect.